A comment by Golden_Spruce, cross-posted from a Reddit discussion. None of these points are original, but this is the best compilation I've seen of reasons that volunteering isn't always as positive as it might seem.

Of course, many volunteers clearly do a great deal of good. But I appreciate this charity's-eye view of the potential downsides of volunteer management, and I plan to think about the likely costs if I consider volunteer work in the future.

Summary

Volunteers can be a major asset to charities, but they also cost a lot of time and resources to manage, even if they are competent and dedicated. 

The costs of volunteer management

Note: I made some slight formatting/copyediting alterations for clarity.

I'm a huge proponent of volunteerism for a lot of reasons, and I think it's awesome that you do it! As a perspective-enhancer, I'll add something for you and others to think about when it comes to volunteering:

Volunteers are a huge asset, but also a big cost to an organization. And in many cases, they are a net cost. But most places consider it an important investment in their PR and in getting people involved in and caring about the work. And I really don't mean to discourage you - it's not a net disadvantage to have volunteers. It's just not free.

There are free/cheap/cost-effective ways to manage volunteers, but a lot of those options involve time, and that usually means paid staff (and donors hate paying for staff so...). Sometimes, volunteers can manage other volunteers, but this isn't always efficient.

Here's a list of the costs of volunteers:

Recruiting

Some organizations don't need to advertise for volunteers, they just come pouring in, far more than you could ever effectively use. But many volunteer positions need to be promoted and advertised just like jobs - everyone wants to play with the puppies, no one wants to clean up the frozen poop in the outdoor play area. It takes time, effort and money to recruit.

Screening

Not all volunteers are appropriate for all jobs. Depending on the position, you may have to interview volunteers (time), do reference checks, submit for criminal record checks and vulnerable sector, if they're volunteering to be your treasurer you may be doing a credit check, testing, etc.

Matching

This is different from screening, because not only do they have to be appropriate for certain jobs, they also have to want to do them or else they won't come back. So you ask them their interests and skills, and where they want to help, and then you expend time and resources to make that fit work. Or you shoehorn them into something and then they inevitably quit and you have the cost of turnover.

Training

All volunteers need training. You might be a fairly competent person who can figure things out, but I guarantee you that 90% of volunteers who come in are worse than useless without very clear and specific training. This means developing policy, training materials (PowerPoints, videos, handouts) or staff dedicating time to hands-on training. And then all those things need to be updated, and you have to keep track of which volunteers have done which training.

Insurance

You're often not allowed to have volunteers without paying extra liability. And they often have to be covered by [workers' compensation] just the same as employees, and someone has to be trained in how to deal with incidents so you don't have lost-time, just like employees. And heaven help you if you don't have insurance and a volunteer gets hurt or hurts a vulnerable client. You can't afford the costs or the reputational damages.

Capital/equipment costs

Volunteers are harder on equipment and supplies than staff. Anyone who works in an office knows that "communal scissors" have to be replaced much more often than ones you keep at your desk. They go missing, they get broken, someone cuts wet cement with them...I don't know how, but the replacement costs for stuff that volunteers use is high. We were doing a clean up one time and two volunteers chucked their high-vis vests and work gloves in a dumpster on the way back - I guess they thought they were one-time use? We didn't think we had to clarify that...but...see "Training".

Reporting

Everyone (foundations, accreditation programs, donors) wants to know so many stats about your volunteers. How many are there, how many hours did they work (now you have to invest in time tracking, extra training and enforcement to make sure the volunteers know how to track their time), which percentage were mandated volunteers, which percentage were corporate volunteers, etc.

If they are doing mandated volunteerism (i.e. the need hours to graduate, or their parole relies on community involvement, or it's part of their disability program), you have to do specialized reporting for all of those things too, and fill out paperwork like mad.

No-shows

If volunteers are scheduled to do some essential function and they don't show up...you have to dedicate staff time to urgently drumming up another volunteer ASAP or pay staff to do it. This leads to organizations giving actual essential work to staff to start with, with less important jobs going to volunteers if they show up. When Meals on Wheels drivers call in sick, you can't just...not do those routes...staff who have whole other jobs to do...have to go and do that work now. And then they come back and work paid or unpaid overtime to get their actual job done. Very few volunteers are interested in being on-call for last-minute no shows. I worked for a seniors centre; you would think retired people would be interested and available at the drop of a hat to fill in, but that is absolutely not the case.

Volunteer appreciation

Many volunteers say "I don't need an org to spend money on me, I don't expect anything", but they absolutely will leave if you don't at least verbally thank them. And, sure, thanking people is "free" - but the effort of thanking dozens or hundreds of people a day (and investing emotional energy into doing this, even if it wasn't actually that helpful, and it caused more work for you than it saved and you just don't feel like being thankful today). 

I don't mean to sound bitter and ungrateful, but if you've worked in a customer service environment, you know that this takes a real actual toll. You can never have a bad day with a volunteer. And that is only the "cost" for free stuff - many volunteers expect, for example, coffee or water.

Another piece of this is being a reference and writing reference letters. Many people volunteer to boost their resume or get a reference. And it takes, surprise surprise, a lot of time to do those things for folks.

And doing a little more than all of that is investment in retention (i.e. having a year-end appreciation for volunteers, or writing them a thank-you note), which leads to the next cost:

Turnover

Volunteer turnover is much higher than staff turnover. Every time a volunteers goes through the whole process up to here and then quits and we have to start over, you incur all these costs again from scratch. This is why many times volunteers cost more than they "save". So you invest in trying to retain them.

Termination

Yep, even firing a terrible volunteer isn't free. This still has to be done thoughtfully and following the right channels, because a disgruntled volunteer can be hugely damaging to your reputation. And then you have to spend the time to document it all, update your volunteer records.

Many organizations do the bare minimum, or don't do these things well (because they often pay minimum wage, and as you can see, this is the work of a skilled HR person, and skilled HR people do not want to do this work). If you're lucky, your organization might spend money on a good database/software for tracking some of this stuff. If you're not lucky, you're trying to do it with paper files or Excel.

Again, this isn't a pity party, or trying to discourage people from volunteering. Volunteers are a sustaining life force to an organization, bringing energy and optimism when staff have none left to give. But they are not free.